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The optional "pcre" map type allows you to specify regular expressions with the
PERL style notation such as \s for space and \S for non-space. The main
benefit, however, is that pcre lookups are often faster than regexp lookups.
This is because the pcre implementation is often more efficient than the POSIX
regular expression implementation that you find on many systems.

A description of how to use pcre tables, including examples, is given in the
pcre_table(5) manual page. Information about PCRE itself can be found at http:/
/www.pcre.org/.

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These instructions assume that you build Postfix from source code as described
in the INSTALL document. Some modification may be required if you build Postfix
from a vendor-specific source package.

Note: to use pcre with Debian GNU/Linux's Postfix, all you need is to install
the postfix-pcre package and you're done. There is no need to recompile
Postfix.

In some future, Postfix will have a plug-in interface for adding map types.
Until then, you need to compile PCRE support into Postfix.

First of all, you need the PCRE library (Perl Compatible Regular Expressions),
which can be obtained from:

    ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/.

NOTE: pcre versions prior to 2.06 cannot be used.

In order to build Postfix with PCRE support you need to add -DHAS_PCRE and a -
I for the PCRE include file to CCARGS, and add the path to the PCRE library to
AUXLIBS, for example:

    make -f Makefile.init makefiles \
        "CCARGS=-DHAS_PCRE -I/usr/local/include" \
        "AUXLIBS=-L/usr/local/lib -lpcre"

Solaris needs run-time path information too:

    make -f Makefile.init makefiles \
        "CCARGS=-DHAS_PCRE -I/usr/local/include" \
        "AUXLIBS=-L/usr/local/lib -R/usr/local/lib -lpcre"

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  * When Postfix searches a pcre: or regexp: lookup table, each pattern is
    applied to the entire input string. Depending on the application, that
    string is an entire client hostname, an entire client IP address, or an
    entire mail address. Thus, no parent domain or parent network search is
    done, "user@domain" mail addresses are not broken up into their user and
    domain constituent parts, and "user+foo" is not broken up into user and
    foo.

  * Regular expression tables such as pcre: or regexp: are not allowed to do
    $number substitution in lookup results that can be security sensitive:
    currently, that restriction applies to the local aliases(5) database or the
    virtual(8) delivery agent tables.

